You can select a picture category from the list in the yellow box (animals, events etc.)

You can search for a particular word or phrase in the information text associated with the pictures, or the 5-character picture reference if it's one you've looked at before and you happen to know it

You can select a document category from the list in the green box (animals, events etc.)

You can search for a particular word or phrase in the document using the clever little Google © search box